Anodic protection against corrosion



Sept. 28, 1965 u. HUTCHISON ETAL 33083925 moms PROTECTION muusr connosIoN Filed Jan. 7, 1960 I s Sheets-Shoat 1 2s 25 I8 I 0c l 22:54: 5 i

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l 24 CONTROLLER ANODE CURRENT OR RATE OF CORROSION E A q (VOLTAGE VOLTAGE AT 4ec AT 468) o w '5 -IO -l5 -20 fVOLTAGE AT 488 MERLE HUTCHISON "'OLEN L. RIGGS, JR.

JOHN D. SUDBURY zymvrons B z 4',

ATTORNEY ANODIC PROTECTION AGAINST CORROSION 3 Shoots-Shoat 2 Filed Jan. 7, 1960 MERLE HUTCHISON OLEN L. RIGGS, JR.

SUDBURY INVENTORS JOHN D.

ATTORNEY p 9 M. I-IuTcI-usbN ETAL 3,208,925

ANODIO PROTECTION AGAINST CORROSION Filed Jan. 7, 1.960 I 3 Sheets-Sheet 3 I LINE I I I I 48 I I o.c. I POWER I sounc: I I I I I I 24 [.76 5 mwmouss so '62 50 6| I I 26 I l8 I I 0 I I 1 I. '6 v I I I I LNE 2 II I 7- I I I 22 4B o.c. l0

upowsn fg puncs I I I I I I I I 1 24 Fla 6 CONTROLLER M ERLE 'HUTCHISON OLEN: L. RIGGS, JR.

JOHN G. SUDBURY INVENTORS ATTORNEY oil industry men 18 sufiiciently indicative of the rate of United States atent 3,203,925 ANODIC PROTECTION AGAINST CORROSION Merle Hutchison, Olen L. Riggs, Jr., and John D. Sudbury, Ponca City, Okla., assignors to Continental Oil Company, Ponca City, Okla., a corporation of Delaware Filed Jan. 7, 1960, Ser. No. 1,136 28 Claims. (Cl. 204-147) This is a continuation in part of co-pending application, Serial No. 777,499, filed December 1, 1958, now abandoned.

This invention relates to an improved method and apparatus for the protection of metals against corrosion by anodic passivation.

As it is well known in the art of corrosion control, the corrosion of many metals may be prevented or seriously curtailed by cathodic polarization. For example, in the many pipelines are protected against corrosive action of the soil in which they are laid by cathodic polarization. Cathodic protection is very useful in the substantially constant adjustment of the anodic current, which in turn requires complicated and expensive control Metals which may be protected by the method of this invention include, by way of example and not by way of limitation, mild steel, all types of stainless steel, titanium or alloys of titanium, hafnium and zirconium.

Examples of a few of the corrosive liquids for which this invention is adaptable include tap tion; organic acids; and inorganic acids such as sulfuric (including oleum), nitric, and phosphoric a ids.

An important object of this invention is other object of this invention is to provide a novel An method of, and apparatus for, controlling the anodic current in a corrosion control system using anodic polarization.

A further object of this invention is to reduce the comexity and cost of equipment required in ananodic polari zation system of corrosion control.

Another object of this invention is to solutions containing chlorides.

And another object of this invention is to improve prodhandle corrosive vessel containing construction and service life.

Other objects and advantages of the invention will be detailed description, when evident from the following read in conjunction with the accompanying drawings which illustrate our invention.

FIG. 1 is a schematicdrawing illustrating a practice of this invention.

FIG. 2 is a curve illustrating variations of anodic cur-' nt, 0 of the corrosion rate of a stainless steel subjected to an acid solution at various potentials of the stainless steel.

' FIG. 3 is a schematic wiring diagram of a suitable an odic current controller of this invention.

FIG. 4 is a curve illustrating the response of the voltage amplifier of the suitable anodic current controller at vessel being protected.

FIGS. 5 and 6 are schematics illustrating other methods of switching the anodic passivation circuit.

Referring to the drawings in detail, and particularly ent operating conditions; furthermore, the presence of iron in the product as a result of color products and troublesome emulsions.

In accordance with the present invention an inert elec- 3,208,925- Patented Sept. 28, 1965 corrosion may result in oflf mized, by anodic polarization.

' position than the material a from coming'into direct nected by a conductor 20 to vessel 10. It will thus be'seen -that vessel functions as an anode and electrode 14 afunctions as a cathode.

As it is well known in the art, and as previously indicated, the rate ,of corrosion of the type being considered of the specimen being corroded, be prevented,or at leastmini- The potential of the is determined with respect to a referenceelectrode located in the E.M.F. tableat a lower or more noble of the specimen. When the specimen is made the anode of an electrochemical cell, the potential of the specimen shifts in the more noble direction. When this shift is of sufficient magnitude, the corrosion stops and it is said. that the specimen has become passive. In this connection it should be .noted that the sign of the potential difference between the specimen and the standard electrode is of no consequence insofar as the nobility (the relative position in the E.M.F. table) of the specimen is concerned, i.e., the sign of this potential difference may be positive or negative or change as the specimen is made more noble. We have found that when theanodic current is stopped, the potential very slowly shifts in the-less noble direction and that the passivity of the specimen remains anodic current is stopped.

It can be shown that the rate of corrosion varies with the potential between the specimen and one type of standard electrode (calomel cell) in the manner illustrated in FIG. 2,v wherein E is the potential of the calomel cell with respect to the specimen. This potential will be sometimes hereinafter referred to as the noble potential or voltage. As shown in this drawing, the rate of corrosion invaries with the potential and'the corrosion may specimen 4 portion of the curve midway between pointsAand B. At this time it is believed that an insoluble fllm has-formed on that portion of the inner surface of vessel 10 opposite solution 12 and no appreciable corrosion is taking place. Controller 24 then opens switch 28 andremovesthe'potential between vessel 10 and the cathode v14. Upon cessation of the anodic current, the potential between vessel 10 and standard electrode 22 creases in a negative direction in' 'the':exarnple illustrated) and vessel 10 becomes less'noble, probably by a gradual deterioration of the insolublefilmon the-inner surface of for an appreciable period after the v I creases at a substantial rate; and then decreases as the noble volt-age approaches zero. As the noble voltage increases in a negative direction, the corrosion rate decreases to a low level for an appreciable range of noble voltages and then increases rapidly again. Thus, as long as the noble voltage is maintained in the range between points A and B of FIG. 2,' which may be referred to as the positive minimum potential area, a minor amount of corrosion takes place.

To measure and control the potential of vessel 10 (FIG. 1) we provide a standard electrode22 communicating with solution 12, along with a suitable controller 24 connected across electrode 22 and vessel 10. Standard electrode 22 may be of any suitable type, such as a calomel cell, a silver-silver chloride cell, a copper-copper sulfate cell or a hydrogen cell, and is connected to the solution 12 by a suitable electrolytic bridge 26 to prevent the cell contact with solution 12 and either affecting operation of the cell or dilution of solution 12. While an electrolytic bridge 26 is shown communicating the standard cell to be understood by those skilled in the art that any well known means may be used to electrically couple the standardcell to solution 12; Bridge 26 must be an ionic conductor and may be either:vv a liquid, such as a KCL solution, orga solid, such as'si-lver chloride. "Controller 24 functions to operate-a switch ,28 in conductor 18'to periodically impose a uniform potential'between vessel 10 and cathode 14 in accordance withtheipotential between vessel 10 and standard electrode 22,.i.e,, the noble voltage;

In accordance with the method of this invention, switch 28 is closed to pass a current-from vessel 10 through solution 12 to inert electrode, until vessel 10 becomes pacified and the rate of corrosion has decreased to a low level. It may be noted that this initial passage of anodic current varies the rate of corrosion in the manner il-.

lustrated in FIG. 2 until the potential between vessel 10 and standard electrode 22 reaches a level between points A and B in FIG. 2, and preferably on theflat minimum 22 with the solution 12, it is vessel 10.

Controller 24' riibnitors variations in;the potential between vessel 10 and standard electrode. 22. When this noble voltage reaches a level close to po t A in FIG. 2, controller 24 closes switch 28-to again impose a potential between vessel 10 and cathode 1'4'and'induces another anodic current. Whereupon,.the potential between vessel 10 and standard electrode 22 again increases to within the vicinity of the flat minimum portion of the curvebe-' tween points A and Hot FIG. 2,:andthe7controller again opens switch 28. This 'step wise procedure is repeated to provide a minimum of corrosionof vessel. 10. The lasting power of the corrosion protectionv (when the anodiccurrent is stopped) ranges from a few seconds immediately following formation passivity has been established from eight to twenty-four hours.

It can to obtain and to maintain passi the same manner as the corrosion rate illustratedrby the curve in FIG. 2.. In other'words, as the specimen or also be shown that the current, density'required vessel is being passivated, rather highcurrent densities,

are required. Howevehoneej the specimen-is passivated,

the passivity may be maintained with :a very'small current density. Thus, the powerorenergy required to maintain corrosion protection by-the present minimum. The data .in the ty is established whenvarious steel samples-are immersed Current Density Weight Losses in Grams 4 Steel Type and No. A.I.S.I.

p (a) l (b) 1 Before After Paml vlty Psssivlty Current density to'passlvate sample (mtlliampereslcmfi). 1 (1;) Current: density to maintain passivity (microsmperes/omfi). 8 Test conducted for 24 hour duration; Sample size 17 sq. cm. surface area.

In a preferred embodimentgof'this invention, and as shown in FIG. 3, controller 24 generally comprises a voltage amplifier 30'controlled'by a reference section 32; and an isolation section or buffer 34 for operating a power amplifien36" without affecting amplifienSO. Power is supplied to the preferred controller 24at various potentials through junctions 38, 40, 42 and. In one embodiment, junction 38 is at -12volts D.C., junction 40 is at -15 volts D.C., junction 42 is at 4-2 volts.D.C. and junction 44 is at ground.

Voltage amplifier 30 comprises two transistors 46 and 48 having their emitter contacts 461?. and 48E connected through a-resistor 50 to junction 42 and through a resistor 52 to junction 44. Collector contact 460 of transistor 46 is connected directly to junction 40 andcollector contact 480 of transistor 48 isconnected'through a resistor 54 to junction 40.

gradually changes (dcof passivity to several'days when ty varies in substantially invention is at a y following table illustrate the various current densities required before and afterpassivi The base contact 463 of tran-.

electrode 22 and vessel sister 48 is connected to the operating point of amplifier 30,

The reference section 32 comprises a diode 55 connected across junctions 40 and 44 in series with a resistor 56 and shunted by a potentiometer 58 having two series resistors 60 and 62.

tion section 34, and transistor 66 in turn drives a power amplifying transistor 68. The emitter contact 66E of transistor 66 is connected to base contact 68B of transistor 68 and both of tion 40 through resistor 70.

to the voltage at junction 38, no current will flow through the coil of relay 72. However, when the voltage at contact 68E is varied, current will flow through the coil of relay 72.

Primary relay 72 is terpose Power circuit 76 may be easily constructed to derive energy from the common D.C. power source 16 illustrated in FIG. 1. Secondary relay 78 in turn controls the operation of switch 28 in conductor 18 leading from power source 16 to the cathode 14 (see FIG. 1). It will be understood that a single relay may be used if sufiicient power is available for operating the switch 28.

In analyzing and summarizing the operation of preferred controller 24 illustrated in FIG. 3, let it be assumed that it is desired to retain the voltage between standard electrode 22 (a calomel cell) and vessel (a stainless steel vessel) at about 500 millivolts D.C., which is somewhere between points A and B of FIG. 2. Potentiometer 58 is adjusted to provide a voltage of about -500 millivolts D.C. at contact 48B, and potentiometer 64 is adjusted to make the voltage at contact 48C about zero when the voltage at 46B is below -45!) millivolts 28 was interposed in the input side of the placed in operation the potential between standard elec-. trode 22 and vessel 10 is 200 millivolts D.C. At this last voltage (being below --450 millivolts) the voltage at 48C, and therefore at 66B, 66B, 68B

tact of transistor will flow through will be opened generate the anodic current each time the potential bevessel' 10 reaches 450 millivolts D.C., whereby the corrosion rate is maintained between points A and'B on FIG. 2.

Referring to FIG. 5, D.C. source16 is of the type-which requires an external power source current.

e operation of the current is substantially identical with the embodiment shown in FIG. 1, however, switch to prevent the necessity for breaking large currents that would normally be present in controller is well known by those skilled in the art,

FIG. 6 shows another method of substantially switching the power on or off source 16.

ings 60 and 61 are connected in series, and in series with conductor 49. The secondary, or control, winding is connected to any suitable D.C. source, suchas a battery, through a pair of conductors 51 and 52. Switch 28 is interposed in one of the conductors 51 or 52 and serves to interrupt the flow of D.C. through control winding 62.

In operation, with switch 28 open, reactor 50 presents Therefore, the majority of the voltage will be dropped across reactor 50, leaving. only a small portion to be ap: plied to D.C. power source 16. With switch 28 closed, reactor 50 presents a very low impedance to the flow of alternating current. Thus, a very small voltage drop will appear across reactor 50 and a very high voltage will be applied to D.C. power source 16. An additional advantage is obtained by this embodiment. Normally switch 28 (see FIG. 1 and FIG. 5) must break a substantial The potential between the standard electrode 22 andv e v ssel 10 'll power supply 18. Further,'as I flow of' alternating current.

oftentimes, destructive operation of switch =28.

described in the patent as a 7 amount of current and oftentimes it must break this current at an extremely rapid rate. Further, it must operate at many times a second. This tends to seriously reduce the overall life of switch 28. The saturable reactor on the other hand provides adelaying action to the circuit. Thus, when switch 28 operates such that power is supplied to DC. power source 16, the applied power will not build up instantaneously, but will require a finite period of time. This period of time tends to prevent rapid, and I The saturable reactor then tends to stabilize the circuit and substantially improve the overall operation and reliability of the circuit..

While a similar I it should be obvious to those skilled in the art that other forms of saturable reactors-may be interposed between cessfully used to prevent corrosion in a form of saturable reactor is disclosed,

or in series with lines 48 and-49. It should also be ap- 1 parent to those skilled inthe art that switches such as thyratrons or ignitrons or semi-conductor switches could easily be substituted. for the disclosed embodiments. Semi-conductor devices or vacuum tubes or their equivalent, could also be substituted forswitch-28 within the teaching and scope of this invention. g

The following examples are given to illustrate our invention. It will be understood that these examples are illustrativeonly and that our'invention is to be taken as limited only by the scope of the appended claimsi EXAMPLE 1 A 500-gallon tank clad with 304 stainless steel was equipped with the anodic polarization system-as shown in FIG. 1,using a 6-volt lead storage battery as the source of DC. current, a platinumelectrode as the inert electrode, and a calomel cell as'the standard electrode (electrochemically communicating with the 'tank by an agar- KCl bridge), The controller was of the type illustrated in FIG. 3.

troller of the type The corrosive solution employed was a 67 percent aqueous solution of sulfuric acid. The current required to passivate this tank (having a surface area of 70 sq. ft. 'or approximately 10,000 sq. in.) was 4.2 amperes applied for about $5 of a second. tain passivity was approximately 142 milliamperes, which amounted to a current density of about 2 milliamperes per square foot (or about 2 microamp. per cm.'*)."-

The current required to main Without application of the anodic passivation technique,

the build-up of metal in a 67 percent H 80 solution in this tank, in 41. hours, was 560 parts per million of iron, 93 p.p.m of chromium, and 58 ppm. of nickel. When a fresh 67 percent solution of H was placed in this tank, and the, tank was passivated anodically, there was no detectable'build-up of iron, chromium, or nickel, after over 120 hours. Thus, it is apparent that application of this anodic passivation technique reduced corrosion to a negligible amount.

EXAMPLE 2 A 60-gallon pfaudler, lined with 316 stainless steel, was charged with a 115 percent solution of phosphoric acid and nonene in the ratio of 1:4 by weight. (Noneneis amixture of .olefinic hydrocarbons in the C -C range.

This material is most economically derived as a by-product of the polymerization of .propene using a phosphoric acid-kieselguhr catalyst, as illustrated in the patent to oleum as charged to "from 30-5,0 p.'p.m., the additional iron content resulting system was passivated using 5, volts. and a current of, 5 amp. Less than 2 volts and 0.6 amp. were required to maintain passivity. The passive potential range was selected at 45-175 millivolts with respect to the reference electrode. The system was maintained in a passive condition for a period of four days, with only.5 ppm. of

iron dissolved during th1s,penod.i Withoutprotection the same environment caused an iron pick-up of Y p.p.m.

in one-day.

- EXAMPLE 3 The method and apparatus of this invention was .suc-

petrochemical plant manufacting water-soluble xylene and toluene-1pctroleum sulfonates suitable for the formulation of detergents. In thisplant the hydrocarbon is sulfonated with oleum, and-the sulfonic acid product is then neutralized with sodium hydroxide to form a neutral sulfonate.

In theabsence of anodic polarizatiomthe equipment is subject to rather severe corrosion, particularly the 304 7 stainless steel; neutralizer tank, which is subject to severe pitting resulting in the necessity for frequent shut-down for repairs, The mild steel oleum storage tank is subject to mild corrosion due to the presence therein of a small amount of water resulting from condensation of moisture. An oleum measuring tank, hereinafter referred to as a blowcase, is subject to corrosion of a more severe degree than the oleum'st'orage tank.

Oleum storage tank The oleum storage tank was passivated using the basic system of FIG. 1 utilizing a platinum electrode, a conshown in FIG. 3, and a ca-lonrel cell as the: standard electrode (electrochemically connected to the liquid oleum by means of a weeping glass bridge assembly).

Prior to installation and operation of the anodic passivation system;

p.p.rm. At the time of installationof the passivation system, the tank contained about 450 p.p.m. of iron. The thetank contains on the average of from corrosion during storage. In less than one week after anodic protection was commenced the iron concentration dropped to 39-41 ppm a value which was 'maintained continuously for a period of over one year.

Blowcuse The system of FIG; 1 was also employed to passivate the blowcase, using a platinum (standard) electrode consisting of silver coatedwithsilver chloride. 'Thesame controller was used for both the oleum tank and the blowcasqthe controller being connected to the oleum tank at all times except-when the blowcase'was being filled. By means of a switching arrangement the controller was connected to the blowcase while it was being. filled, during which time the oleum tank did not lose its passive condition.

Oleum samples from the blowcase had contained about 550 ;p.p.m. dissolved iron prior to start-up of the-anodic passrvation system. Following start-up the iron concentration was reduced to ppm. after three batch runs. Following an initial two-week period, the iron concentration in the blowcase did not exceed 50 p.p.m..

Neutralization tank The neutralization of,;;sulfonic acid with caustic in- I volves a multi-pH system, which subjects the metal to a range of corroding potentials. Since the anodic polarization technique depends upon the existence of a positive minimum potential area (as shown in FIG, 2) in which a minimum of corrosion occurs, it was necessary to determine the polarizationcurves for both the sulfonic acid and the caustic. -It was found thatthe two curves overla to afford a safe operating range in which the poteniron content in the oleum tank had been as high as 1273 p.p.m., the average being about 500-600 1 electrode and a reference the system was installed, the down-time Just 1: been made to replace the tank,

tial could be maintained prior to and during actual neutralization.

the sulfonicacid, the temperature began to rise, and rose F. At the latter temperarequired to maintain passivity was about 7.5 amp. per sq. ft. 1

Prior to installation of the anodic passivation system, the neutralizer was was substantially reduced, and was only 4 days out of one 3month period of time,

rior to installation of this system, a decision had (2) After passivation the amount of cutf formation was greatly decreased.

(3) Prior to passivation, the sulfonate product was loaded with a fluffy white flocculant matter. After passiva-tion, the product was bright and clear.

(4) The amount of ironin three batches of sulfonate This throwing power is believed to be due to for-matron of an oxide film of high resistance. Once this film has been formed to render the metal surface passive, only small increments of additional current are required to patch the anodically minimizing corrosion of a metallic vessel containing a corrosive solution, said metallic vessel and corrosive solution system having an anodic cathode in the solution until said potential drtference again reaches said first-mentioned level. I

2. A method of anodically minimizing corrosion of a metallic vessel containing a corrosive solution,

, saidrange.

4. The method of claim is constructed of mild steel. 3 5. The method of claim 4 wherein the corrosive solu tion is a caustic solution.

6. The method of claim 4 tion is a sulfuric acid solution.

7. The method of claim 4 wherein the corrosive solution is a phosphoric acid solution.

8. The method of claim 4 wherein tion is nitric acid solution.

9. The method of claim 4 wherein the corrosive solution is a liquid fertilizer.

.10. The method of claim 3 is constructed of stainless steel.

11. The method of claim 10 wherein thecorrosive solu-, tion is a caustic solution. 12. The method of claim 10 wherein the corrosive solution is a phosphoric acid solution.

13. The method of claim 10 wherein the corrosive solution is a sulfuric acid solution.

3 wherein the metallic vessel wherein the corrosive soluthe corrosive soluwherein the metallic vessel said metaltion is a liquid fertilizer. 7

claim 3 wherein the metallic ves- 14. The method of claim 10 wherein the corrosive 1 16. The method of selis constructed of titanium.

17. The method of claim 16 wherein the corrosive solution is a causticsolution. i

18. The method of claim 16 wherein the corrosive solution is a sulfuric acid solution- 19. The method or clai'rfn 16 wherein the corrosive solution is a phosphoric acid'solution.

20. The method of claim 16 whereinthe corrosive solution is a nitric acid solution.

21. The method of claim 3 wherein the metallic vessel is constructed'of an alloy of a" metal selected from the group consisting of titanium, hafnium, and'zirconium.

22. The method of claim 21' solution is a caustic solution.

23. The method of claim 21 wherein-the corrosive solu tion is a sulfuric acid solution.

24. The methodof claim 21 wherein the corrosivejsolution is a phosphoric acid solution.

25. The method of claim 21 wherein the corrosive solution is a nitric acid solution.

wherein the corrosive .26. The method defined in claim 1 wherein the vessel 'is stainless steel and said inert electrode is platinum.

defined in claim- 27. The method 6 said standard electrode is a calomel is stainless steel and cell.

28. The method defined in claim 1 characterized further in placing the standard electrode in communication with the solutionby an electrolytic bridge.

References Cited by the Ennrnlner UNITED STATES PATENTS 2,221,997 '11/40 Polin 2,483,397 10/49 Bonner 204-4196 2,576,680 11/51' Guitton 204-447 2,584,816 2/52 Sands 204-231 2,759,887 8/56 Miles 204-196 2,764,543 9/56 Comstock et al. 204-231 2,803,797 8 /57 Cowles 204-196 2,848,658 8/58 Mitchell -1 3l7--148.5 2,903,405 9/59 Sabins ..-204-196 2,918,420 12/59 Sabins- 204231 2,982,714 5/61- Sabins 204-196 2,998,371 8/61 Sabins 204-196 3 ,009,865 11/61 Mueller et al. 204-147 JOHNI'L'MACK, Primary Examiner. JOSEPH REBOLD, JOHN R. SPECK,

MAN, Examiners.

1 wherein the vessel MURRAY TILL- I 

1. A METHOD OF ANODICALLY MINIMIZING CORROSION OF A METALLIC VESSEL CONTAINING A CORROSIVE SOLUTION, SAID METALLIC VESSEL AND CORROSIVE SOLUTION SYTEM HAVING AN ANODIC POLARIZATION CURVE WHEREIN THE METALLIC VESSEL AND CORROSIVE SOLUTION SHOWS A MINIUMUM CURRENT REGION COMPRISING PASSING DIRECT CURRENT FROM THE VESSEL TO AN INERT CATHODE IN THE SOLUTION UNTIL THE POTENTIAL DIFFERENCE BETWEEN THE VESSEL AND A STANDARD ELECTRODE WHICH IS N ELECTROCHEMICAL COMMUNICATION WITH THE CORROSIVE SOLUTION REACHES A LEVEL CORRESPONDING TO A PREDETERMINED MAXIMUM NOBILITY OF THE VESSEL, THEN STOPPING THE FLOW OF SAID CURRENT UNTIL SAID POTENTIAL DIFFERENCE REACHES A LEVEL CORRESPONDING TO A PREDETERMINED MINIUMUM NOBILITY OF THE VESSEL, THEN AGAIN PASSING SAID CURRENT FROM THE VESSEL TO THE CATHODE IN THE SOLTION UNTIL SAID POTENTIAL DIFFERENCE AGAIN REACHES SAID FIRST-MENTIONED LEVEL. 